1. FIELD OF THE INVENTION
The present invention relates to a thin film electroluminescence device having dual insulation layers, and more particularly to an improvement of a structure at the interface between a transparent electrode and first insulation layer.
2. DESCRIPTION OF THE PRIOR ART
A conventional thin film electroluminescence device having dual insulation layers is so arranged that a transparent electrode 22 is deposited on a transparent base plate 21 and a first insulating layer 23 is deposited on the transparent electrode 22. On the first insulating layer 23 a light emitting layer 24, a second insulating layer 25 and a back electrode 26 are laminated, as shown in FIG. 1, and a power supply 27 is connected across the back electrode 26 and the transparent electrode 22. As the material of the first insulating layer 23, there has been used Ta.sub.2 O.sub.5 (tantalum oxide) as disclosed in the Japanese Patent publication (unexamined) Tokkai sho-58-157887 since Ta.sub.2 O.sub.5 has a high dielectric constant and is stable under a high electric field, therefore Ta.sub.2 O.sub.5 shows excellent properties as the insulating material.
However, in the case when a Ta.sub.2 O.sub.5 layer is deposited as the first insulating layer 23 on the transparent electrode 22, the resistance of the ITO (indium tin oxide),of the transparent electrode 22 increases and it becomes difficult to provide bright electroluminescence elements. It is noted that oxygen diffuses into the Ta.sub.2 O.sub.5 layer at the time of forming the ITO layer, such that the diffusion of the oxygen causes an increase in the resistance of the transparent electrode 22.
It is known that the increase in the resistance of the ITO layer can be prevented if the, Ta.sub.2 O.sub.5 layer is deposited after a SO.sub.2 layer is deposited according to the existing literature, for example by Y. Simizu, and T. Matsudaira, IEEE 1985, Inter. Display Research Conf. P101. However, to employ the above technique may invite the problems whereby a high drive voltage is required because the dielectric constant of the material used therein is too low.
The present inventors have tried to form the Ta.sub.2 O.sub.5 layer under a low temperature. In this technique, although the increase in the resistance of the ITO layer can be suppressed, there tends to result in an oxygen deficiency in the Ta.sub.2 O.sub.5 layer, whereby it is difficult to form a Ta.sub.2 O.sub.5 layer in which current leakage is small.